A protocol view controllers should conform to for enabling the view controller to be used as a drawer header view controller.
The drawer header transition to top ratio: zero represents the drawer being fully displayed as part of the content, one represents the drawer being fully displayed as a top header.
moves between 0 to 1 as the transition of the header view transforms from being above the content to becoming sticky on the top. It is 0 when the drawer is above the content and starts changing as the header view expands to cover the status bar and safe area based on the completion rate. It is 1 once the header finishes its transition to become sticky on the top and it’s height is at the size of its preferredContentSize + the safe area.